1. Technical Field
The present invention relates to a starter electromagnetic switch for opening and closing a main contact provided on a starter motor circuit to thereby switch on and off motor energization current.
2. Related Art
A known starter electromagnetic switch, as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-177160, includes, as shown in FIG. 5, a solenoid SL configured to form an electromagnet through energization of a coil 100 to drive a plunger 110 by means of an attractive force of the electromagnet, and a movable contact 130 attached to an end of a plunger rod 120 secured to the plunger 110. The movable contact 130 is arranged in opposition to a pair of fixed contacts 140 electrically connected to a starter motor circuit. The movable contact 130 moves in the axial direction of the plunger 110 integrally therewith in response to ON/OFF operation of the solenoid SL (i.e., excited/unexcited state of the coil 100) to thereby electrically connect and disconnect the pair of fixed contacts 140.
The electromagnetic switch disclosed in Japanese Patent Application Laid-Open Publication No. 2006-177160 includes a contact compartment 160 on the anti-plunger side of a fixed iron core 150 to be magnetized by the electromagnet, in which compartment the pair of fixed contacts 140 and the movable contact 130 are arranged. More specifically, the fixed iron core 150 has a cylindrical bore 170 located radially centrally therein. The plunger rod 120 extends through the bore 170, and the end portion of the plunger rod 120 resides in the contact compartment 160. In addition, a contact pressure spring 180 is provided on the outer circumferential periphery of the plunger rod 120 to bias the movable contact 130. To install the contact pressure spring 180 inside the inner diameter of the bore 170 without interference with the fixed iron core 150, the inner diameter of the bore 170 is set greater than the outer diameter of the contact pressure spring 180. In the above configuration, however, a spatial gap between the inner diameter of the cylindrical bore 170 in the fixed iron core 150 and the outer diameter of the plunger rod 120 may lead to fluid communication between a plunger movement space 190 in which the plunger 110 can axially move (hereinafter referred to as a plunger compartment) and the contact compartment 160, so that moisture is prone to intrude from the plunger compartment 190 into the contact compartment 160.
Additionally, as shown in FIG. 6, a step is provided on the outer circumferential periphery of the plunger 120 such that the plunger rod 120 is formed of a thick portion on the plunger side and a thin portion on the anti-plunger side along the axial direction and the outer circumferential periphery of the thick portion of the plunger rod 120 is in sliding contact with the inner circumferential periphery of the cylindrical bore 170. In this configuration, there is substantially no spatial gap between the outer circumferential periphery of the thick portion of the plunger rod 120 and the inner circumferential periphery of the cylindrical bore 170, which can prevent moisture from intruding from the plunger compartment 190 into the contact compartment 160 during transition from an inactive state (in which the solenoid is OFF) to an active state (in which the solenoid is ON).
During transition from the active state to the inactive state, however, the plunger 110 will be pushed back to the left (as viewed in the drawings) by a restoring force of the return spring 200, which may cause a substantial negative pressure to be produced in the contact compartment 160.
In the presence of moisture in the plunger compartment 190 (e.g., on a surface of the plunger 110 or on an outer peripheral surface of the plunger rod 120), the negative pressure produced in the contact compartment 160 may cause the moisture to be sucked from the plunger compartment 190 into the contact compartment 160. Hence, for example, when the outside temperature falls below freezing, the moisture sucked into the contact compartment 160 may freeze to contact surfaces of the fixed contacts 140 and/or the movable contact 130. This may lead to conduction defects between these contacts during operation of the electromagnetic switch. To prevent such conduction defects, ice produced on the contact surfaces has to be broken by contact bombardment upon contact of the movable contact 130 with the fixed contacts 140, which requires increasing the attractive force of the solenoid to thereby enhance the contact bombardment upon contact.
Further, in the electromagnetic switch as shown in FIG. 6, the presence of two slidable contact portions, i.e., a slidable contact portion of an outer circumferential periphery of the plunger 110 and a slidable contact portion of an outer circumferential periphery of the plunger rod 120, may cause prying of the plunger 110 and the plunger rod 120 upon activation of the solenoid SL when the plunger 110 and the plunger rod 120 are off-center from each other. Such prying of the plunger 110 and the plunger rod 120 may increase slidable resistance during the plunger being attracted, which may lead to conduction defects between the fixed contacts 140 and/or the movable contact 130. Normal operation of the electromagnetic switch even in the presence of increased slidable resistance due to prying of the plunger and the plunger rod requires the attractive force of the solenoid SL to be increased.
Conventionally, increasing the attractive force of the solenoid SL may lead to a disadvantage that an outer diameter and weight of the electromagnetic switch will be increased.
In consideration of the foregoing, it would therefore be desirable to have a starter electromagnetic switch capable of minimizing intrusion of moisture from a plunger compartment into a contact compartment to reduce an attractive force of a solenoid and thereby reduce both size and weight of the switch.